1. Field of the Invention
This invention relates generally to a system for controlling the impedance of a robotic manipulator and, more particularly, to a scheme for multiple prioritized operational space impedances at different points on a robotic manipulator.
2. Discussion of the Related Art
Arms and manipulators are essential to many robotic tasks. For example, they are essential to robotic assembly tasks. Many robot arms and manipulators designed for factory use are stiff, high-impedance arms that operate using position control systems. However, the ability to control forces applied by the robot on the environment is also useful in assembly tasks. In particular, it is useful to be able to control the impedance of the manipulator, i.e., the relationship between applied environmentally forces and manipulator position.
Active impedance control of robot arms and manipulators has been studied and different approaches have been proposed. One of the earliest approaches was a strategy for controlling end-effector stiffness in Cartesian coordinates. Assuming the existence of a low-level torque controller, this approach calculated the joint torque corresponding to a Cartesian space error given a desired Cartesian space end-effector stiffness. This can be considered to be a system for achieving a desired linear zero-order impedance. This approach was later expanded into a system for controlling second-order linear impedance. In this approach, the control system attempts to adjust not only the stiffness, but also the manipulator damping and the inertia.
Another body of work relevant to the current invention is multiple-priority velocity and acceleration control. It is well known that because a robot manipulator has enough actuated degrees of freedom, a manifold of joint configurations may exist that place the end effector in the same position and orientation. This property is known as the kinematic redundancy of the manipulator and has been used primarily to enable robots to achieve multiple velocity or acceleration objectives at once. For example, with a seven degree-of-freedom (DOF) humanoid manipulator, it is possible to place the manipulator end-effector in a particular pose while also adjusting the position of the elbow as desired. Typically, position objective are assigned priorities with lower-priority position objective operating subject to higher-priority objectives.